feat: add frontend subscription page - M4 (refs #55)

frontend/src/features/subscription/CLAUDE.md

@@ -0,0 +1,33 @@
+# frontend/src/features/subscription/
+
+Subscription and billing management feature with Stripe integration.
+
+## Files
+
+| File | What | When to read |
+| ---- | ---- | ------------ |
+| `README.md` | Feature overview and API integration | Understanding subscription flow |
+
+## Subdirectories
+
+| Directory | What | When to read |
+| --------- | ---- | ------------ |
+| `types/` | TypeScript types for subscription data | Working with subscription types |
+| `api/` | Subscription API client calls | API integration |
+| `hooks/` | React hooks for subscription data | Using subscription state |
+| `components/` | Reusable subscription UI components | Building subscription UI |
+| `pages/` | Desktop subscription page | Desktop implementation |
+| `mobile/` | Mobile subscription screen | Mobile implementation |
+| `constants/` | Subscription plan configurations | Plan pricing and features |
+
+## Key Patterns
+
+- Desktop: MUI components with sx props
+- Mobile: Tailwind classes with GlassCard
+- Stripe Elements for payment methods
+- React Query for data fetching
+- Toast notifications for user feedback
+
+## Environment Variables
+
+- `VITE_STRIPE_PUBLISHABLE_KEY` - Required for Stripe Elements initialization